1. Field of the Invention
The present invention relates to a method for fabricating a memory device. More particularly, the present invention relates to a method for fabricating a read only memory device.
2. Description of Related Art
The read only memory device provides the non-volatile property, wherein the stored information is retained even electrical power is interrupted. As a result, the read only memory device is incorporated into many electronic products to maintain a normal operation for the products. The mask read only memory device is the most fundamental type of read only memory device. A typical mask ROM device uses a channel transistor as the memory device. The programming of a mask ROM device is accomplished by selectively implanting ions to an identified channel region. By altering the threshold voltage, the control of the “on” state and the “off” state of the memory device is thus achieved.
A typical mask read only memory device comprises a polysilicon word line crossing over a bit line. The region under the word line and between the bit lines is the channel region of the memory device. Whether or not ions are implanted to the channel region determines the storage of the binary digit of either “0” or “1”, wherein the implanting of ions to the identified channel region is known as code implantation.
In general, when a code implantation process is performed on a mask read only memory, a patterned photoresist layer that exposes a pre-coding region is formed with a photomask. An ion implantation process is then performed to implant dopants to the pre-coding region in the substrate under the stacked gate structure. Usually, as required by the circuit design, an isolated pattern region and a dense pattern region are formed on a same photomask. An overexposure and an underexposure in the isolated pattern region and in the dense pattern region often occur during the transferring of a mask pattern due to the optical proximity effect. The accuracy of the critical dimension of the transferred pattern is thereby affected. Consequently, a misalignment would result as dopants are being implanted to the identified channel region of a mask read only memory device during the code implantation process. Information that is being stored in the memory cells of the memory device is thus erroneous to adversely affect the operational function of the memory device and the reliability of the products.
Conventionally, to correct the non-uniformity of pattern in the dense pattern region and the isolated pattern region of a coding mask of a mask read only memory device, the optical proximity correction technique is employed to design a mask with a special pattern. However, this type of mask with a special pattern, not only increases the manufacturing time, it also complicates the manufacturing process and increases the manufacturing cost. Further, after the fabrication of the photomask is completed, it is rather difficult to debug the defects in the pattern of this type of photomask.